Blasting cap



June 13, 1961 J. P. IRELAND, JR

BLASTING CAP Filed NOV. 6, 1958 INVENTOR JOHN PAUL /RELA/VD, JR

2,987,997 Patented June 13, 1961 2,987,997 BLASTING CAP John PaulIreland, In, Pompton Lakes, N.J., assignor to E. I. du Pont de Nemoursand Company, Wilmington, DeL, a corporation of Delaware Filed Nov. 6,1958,'Ser. No. 772,282 4 Claims. (Cl. 10228) The present inventionrelates to a novel electric blasting cap for use in blasting operations.More particularly, the present invention pertains to an improvedelectric blasting cap which is protected to a great extent frompremature detonation caused by electrostatic discharges at low energylevels.

In the field, electric blasting caps are sometimes subjected toelectrostatic energy which can cause premature detonation of the cap dueto sparking depending upon a variety of circumstances. This isespecially true of blasting caps having a powdered lead styphnateignition composition. A tendency toward premature detonation is, ofcourse, extremely dangerous to personnel who handle caps or who are inthe vicinity when explosive shots are being set up. There are a numberof ways in which electrostatic discharges commonly cause prematuredetonation of an electric blasting cap, as will be more particularlyhereinafter described.

It is an object of the present invention to provide a novel blasting capwhich is to a great extent protected against premature detonationresulting from electrostatic discharge. It is a further object of theinvention to provide such a blasting cap which is simple, convenient andmay be economically fabricated. These and other objects of the inventionwill become apparent from a consideration of the ensuing specificationand claims.

The invention is particularly concerned with electric blasting capshaving a powdered, crystalline lead styphnate ignition composition.Powdered lead styphnate, as commonly used for ignition purposes inelectric blasting caps is very dusty and can fluff up into variouslocations within the blasting cap itself where it may be subjected tocontactwith an electrostatic discharge. Under certain circumstances thelead styphnate dust may flash and propagate to the main ignition chargecausing the cap to fire. I have greatly minimized this danger byproviding atop the lead styphnate ignition composition a thin layer of apowdered substance of such a consistency and density that it tends toact as a cover for the lead styphnate composition to prevent the egressof lead styphnate dust from the ignition area. This powdered cover layerwill permit the bridge wire to pass through it during the manufacturingoperation such that the bridge wire can be embedded in the ignitioncomposition, but in the final assembled cap product, the cover layereffectively insulates the ignition composition from the rest of theshell.

A more complete understanding of the invention may be gained from astudy of the attached drawing which represents a cross-sectional view ofan electric blasting cap prepared in accordance with the presentinvention.

In the drawing, 1 represents the outer metal shell which houses thecomponents of the blasting cap, 2 is a base load, as for example PETN,and 3 represents a primer load which may be lead azide or any othersuitable composition that is commonly used for priming purposes inelectric blasting caps. A plastic capsule 4 s positioned within theshell atop the primer load 3. The capsule holds the lead styphnateignition composition 5-which may contain in addition to the leadstyphnate any of a variety of compounding ingredients, such asmicronized graphite for static dissipation, calcium stearate forimproved fiowability, etc. The cover layer 6 is positioned just on topof and directly adjacent to the lead styphnate ignition composition. Apair of leg wires 7, 7' extend through a rubber plug 8 which seals thetop of the cap. The leg wires have a conventional bridge wire 9 betweenthem which is buried in the ignition composition. The shell and therubber plug are crimped as at 10 and 11 for sealing purposes. The legwires7, 7 are provided with beaded portions 12, 12 which sit in atransverse channel 13 in the rubber plug 8 to provide a harmless pathfor an are which might result from the application of excessive voltageto the cap.

In ordinary electric blasting caps, premature detonation of the capbecause of electrostatic discharge usually results in one of twoprincipal ways: electrostatic discharge from a leg wire to the shell atthe plug-ignition composition interface or a similar discharge betweenthe shell and the beaded portion '12 of a leg wire in the groove 13provided for this purpose. In the former case the discharge can ignitethe ignition composition directly. In the latter case, it may ignite anylead styphnate dust which has drifted up around the leg wires which inturn can propagate to the main body of the ignition composi tion.

The cover layer 6 of the present invention serves to effectively sealoff the ignition composition from the top portion of the cap and thusserves to prevent premature detonation via the aforementionedmechanisms. A wide variety of powdered materials may satisfactorily beemployed to form the cover layer. This layer may be either explosive,combustible, or inert in nature except that, if explosive, it must beconsiderably less sensitive to electrostatic discharge than leadstyphnate powder for it to afford the desired protection. Generallyspeaking, an explosive powder should have an ignition probability ofzero when subjected to an electrostatic energy of 0.060 joule.

The cover layer is preferably formed of a powder having a relativelyfine particle size so that the bridge wire can pass through it into thelead styphnate ignition composition without pulling any of the coverlayer down with it. In this connection, the powdered material shouldhave a nominal particle size not exceeding 60 microns. It is best if thecover layer powder has good compressibility characteristics so that itmay become waferlike when compressed between the ignition composition inthe plastic capsule and the plug. In this manner it eifects a good, highdensity seal between the plug and the ignition composition and will tendto inhibit the formation of an electrostatic spark from occurring atthis interface.

The cover layer should also exhibit good stability to moisture and heat.In this latter connection, the powder should be stable at temperaturesup to about 220 F. For convenience in loading the powdered material intothe cap, the powder should be relatively free flowing in nature.

The cover layer may be electrically conductive in which event it canprovide a direct lead wire-to-shell circuit to afiord a predeterminednon-sparking path for an electrostatic charge. In this connection, theconductivity of the layer may be varied over a wide range. A naturallyconductive cover layer, such as a finely divided metal like powderedcopper, iron, or aluminum may be used or a non-conductive layer may berendered conductive by the addition of finely powdered graphite, or thelike.

For an explosive cover layer, especially good results have been obtainedwith lead nitrate-bis-basic lead-4,6- dinitro-o-cresylate. This is acomplex of a lead nitrate formed by the reaction of a leadnitrate-sodium hydroxide solution with dinitro-o-cresol and is generallyrepresented as follows: (C H N O Pb-2Pb(OH -Pb(NO For-an inert coverlayer, good results have been achieved with 'fin'ely divided diba'siclead phosphite. 'Other explosive compositions are alsoygperableincluding, for example, finepowdered PETN RDX, and black, pow.der.

"The cover layer may. also beformed from,combust'ible materials such assmokeless powder, bismuth, selenium, tellurium, or magnesium mixed withbarium peroxide.

'On the other hand, the cover layer may be essentially inert and in thisconnection such materials as powdered barium peroxide, .cement, andvarious oxides of common metals such as aluminum, zinc, copper, lead,and iron are wholly satisfactory.

The plastic capsule 4 serves as a container for the lead styphnateignition composition and also serves to insulate this sensitive ignitioncomposition from the shell wall.

Further details of this'plastic capsule may be found in pendingapplicationSerial No. 627,539, filed December 5, 1956, in the names ofC, RJJohnson and M. E. Swanson, now U.S. Patent No. 2,878,752, issuedMarch 24, 1959.

In manufacturing caps in accordance with the present invention the capisassembled invthe usual way up to the point where the ignitioncomposition has been inserted. At this stage the thin cover layer isadded and the plug containing the leg wires is ginsertedinto the cap,the bridge wire passing'through the cover layer'and embedding itself inthe ignition composition. The cap is then crimped in the conventionalway.

The cover layer of the present inventionatfords valuable andreliable-protection against premature detonation due to electrostaticdischarge. In-terms of material and fabrication-costs, it is relativelyinexpensive and convenient to install.

The invention is-applicable'to regular electric blasting caps aswell asto delay caps. In the latter case, the cap will'contain asuitable delaytrain between the ignition composition Sand the primer load 3.

The invention has been described in the foregoing specification. It willbe readily 'apparentto those skilled in the art that many changes may bemade in the assembly, compositions, and procedures described withoutdeparting from the scope of the invention. 'It is intended therefore tobe limited only by the following claims.

I claim:

1. An electric blasting cap comprising an outer shell, a'base load atthe bottom of said shell, a priming composition directly adjacent to andjust atop said base load, a plastic capsule containing a powdered leadstyphnate ignition composition above the priming composition, a pair ofleg wires supporting a bridge wire embedded in said ignitioncomposition, a plug member within said shell and positioned at the topthereof supporting said leg wires and sealing the contentsof the .shell,and a separate,

relatively thin cover layer of a compressible powdered substance betweenthe top surface of the ignition composition and the bottom surface ofthe plug member, said powdered substance being substantially lesssensitive to initiation by electrostatic discharge than powdered leadstyphnate.

2. An electric blasting cap .havingan outer shell, which contains inascending order, a base load, a priming composition, and a plasticcapsule containing. powdered, lead styphnate ignition composition, andhavingaplugmember atop the ignition composition which supports ia pairof leg wires mounting a bridge Wire at their lowerend which is embeddedin the ignition composition, the. novel improvement which comprises aseparate, relativelythin cover layer of a compressible powderedsubstance between the top surface of the lead styphnate ignitioncomposition and the bottom surface of the plug member, said powderedsubstance beingsubstantially less sensitive to initiation byelectrostatic discharge. than powdered lead styphnate.

3. An electric blasting. cap as in claim 2 wherein said last-mentionedcompressible powdered substance is itself a detonating explosivecompound.

4. An electric blasting capas in claim 2 wherein said last-mentionedcompressible powdered substance is itself a non-explosive composition.

References Cited in the file of this patent UNITED STATES PATENTSJohnson et a1. Mar. 24, 1959

